The present invention relates to a multiple-sheathed sodium leakage detection apparatus usable in various equipment: e.g. a cooling system piping in which a well of a liquid metal-cooled fast breeder reactor is installed; a nuclear reactor vessel which is bounded by sodium through a well; various testing equipment; and plants that use an electrically conductive fluid.
Sodium is used as coolant in liquid metal-cooled fast breeder reactors, sodium testing equipment, etc. To measure the temperature in the piping for sodium, a well for measuring the sodium temperature (well) is installed directly on the piping, and a thermocouple is inserted into the well, thereby measuring the sodium temperature.
FIG. 1 shows a typical conventional process instrumentation system. A well 2 for temperature measurement is welded to a sodium cooling system piping 1 in which high-temperature sodium is flowing. A sheathed thermocouple 3 is inserted in an inert gas or air atmosphere in the well 2 and connected to compensating lead wires 5 through a terminal unit 4. A detected signal is sent to a process instrumentation system 30 of the plant.
FIG. 2 is an enlarged view showing only the part of the temperature measurement well 2 in FIG. 1. As shown in FIG. 3, which is a horizontal section view cut by the arrow A--A in FIG. 2, the sheathed thermocouple 3 has its element wires 7 disposed in an insulating material filled in a sheath. In the terminal unit 4, the thermocouple element wires 7 and the compensating lead wires 5 are connected together at a wire connection part 6. It should be noted that reference numeral 8 denotes a hot junction of the thermocouple element wires 7.
The conventional sheathed thermocouple of the type described above is an MI cable in which: the sheath material is, for example, SUS316; the thermocouple element wires are a combination of alumel and chromel wires; and the insulating material is magnesium oxide.
Incidentally, the above-described well is caused to vibrate at a portion thereof inserted in the sodium piping 1 by Karman vortex due to the flow of sodium in the piping 1. If the well is not very strong in structure, the vibration may cause the well to break. If the well is broken, sodium in the piping may leak to the outside of the piping through the well. In the case of a well such as that shown in FIGS. 1 to 3, however, when a small amount of sodium leaks out of the sodium piping into the well, it is difficult to detect the leakage of sodium from only a temperature change at the hot junction 8 of the thermocouple element wires isolated in the sheath.
Thus, it is technically difficult in the present art to discover leakage of sodium at the instant when a small amount of sodium has leaked into the well, and there is no effective means to detect a leakage of sodium at an early stage of the leakage when the well welded to the sodium piping is broken by the fluid vibration or the like. Accordingly, it has been demanded to develop a detection apparatus for discovering a small amount of sodium leakage in the well due to a breakage of the well at an early stage of the leakage.